Re: MultiXact\SLRU buffers configuration

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: "Andrey M(dot) Borodin" <x4mmm(at)yandex-team(dot)ru>
Cc: vignesh C <vignesh21(at)gmail(dot)com>, Andrew Borodin <amborodin86(at)gmail(dot)com>, i(dot)lazarev(at)postgrespro(dot)ru, Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru>, Andres Freund <andres(at)anarazel(dot)de>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Gilles Darold <gilles(at)darold(dot)net>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <aekorotkov(at)gmail(dot)com>, Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, pgsql-hackers mailing list <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: MultiXact\SLRU buffers configuration
Date: 2024-04-07 18:41:41
Message-ID: 202404071841.oxm5ojuahd2q@alvherre.pgsql
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On 2024-Feb-03, Andrey M. Borodin wrote:

> Here's the test draft. This test reliably reproduces sleep on CV when waiting next multixact to be filled into "members" SLRU.
> Cost of having this test:
> 1. We need a new injection point type "wait" (in addition to "error" and "notice"). It cannot be avoided, because we need to sync at least 3 processed to observe condition we want.
> 2. We need new way to declare injection point that can happen inside critical section. I've called it "prepared injection point".
> Complexity of having this test is higher than complexity of CV-sleep patch itself. Do we want it? If so I can produce cleaner version, currently all multixact tests are int injection_points test module.

Well, it would be nice to have *some* test, but as you say it is way
more complex than the thing being tested, and it zooms in on the
functioning of the multixact creation in insane quantum-physics ways ...
to the point that you can no longer trust that multixact works the same
way with the test than without it. So what I did is manually run other
tests (pgbench) to verify that the corner case in multixact creation is
being handled correctly, and pushed the patch after a few corrections,
in particular so that it would follow the CV sleeping protocol a little
more closely to what the CV documentation suggests, which is not at all
what the patch did. I also fixed a few comments that had been neglected
and changed the name and description of the CV in the docs.

Now, maybe we can still add the test later, but it needs a rebase.

Álvaro Herrera Breisgau, Deutschland —
“Cuando no hay humildad las personas se degradan” (A. Christie)

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